1. Field of the Invention
The present invention relates generally to a system and method of obtaining measuring characteristic correction factors for a respiratory flow measuring device using a static pressure difference, and more particularly to a system and method of obtaining measuring characteristic correction factors for a pneumotachometer most generally used for respiratory flow measurement.
2. Description of the Prior Art
As well known to those skilled in the art, a pneumotachometer is widely used to examine the capacity of the lungs. The pneumotachometer is constructed to measure the difference between static pressures generated via a fluid resistor in the center of a cylindrical tube using a differential pressure sensor connected to the cylindrical tube. In such a pneumotachometer, when respiratory airflow passes through a fluid resistor, energy is lost due to friction with the fluid resistor, thus generating a differential pressure due to the loss of energy. Accordingly, this device is constructed to calculate respiratory flow by measuring the differential pressure. The pneumotachometer is designed to change the sign of the differential pressure when the direction of the respiratory airflow is reversed, thereby continuously measuring the respiratory flow of a human body, flowing in both directions of exhalation and inhalation. When respiratory flow is measured by the pneumotachometer, the relationship between differential pressure xcex94P and respiratory flow F is regulated by an experimentally-derived characteristic equation. Typically, since a fluid resistor R is represented by a linear function of a flow, the differential pressure xcex94P is expressed by a quadratic function of the respiratory flow F as indicated in Equation [1] which is Rohrer""s equation.                                                                         Δ                ⁢                                  xe2x80x83                                ⁢                P                            =                                                R                  ⁡                                      (                    F                    )                                                  ·                F                                                                                        =                                                (                                                            R                      0                                        +                                                                  R                        1                                            ⁢                      F                                                        )                                ·                F                                                                                        =                                                                    R                    0                                    ⁢                  F                                +                                                      R                    1                                    ⁢                                      F                    2                                                                                                          [        1        ]            
In Equation [1], if coefficients R0 and R1, representing measuring characteristics, are already known, the respiratory flow F can be determined by measuring xcex94P using the following Equation [2].                     F        =                                                                              R                  0                  2                                +                                  4                  ⁢                                                            R                      1                                        ·                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                  P                                                      -                          R              0                                            2            ⁢                          R              1                                                          [        2        ]            
In Equation [2], R0 and R1 are individually determined with respect to both directional respiratory airflows of the human body (inhalation and exhalation).
Meanwhile, since there are two measuring characteristic coefficients R0 and R1 in Equation [1], xcex94P and F are not proportional to each other, so the measuring characteristic coefficients can be determined only when a pair of xcex94P and F are measured at two or more points.
A rotometer is generally used as a device for determining the characteristic coefficients, which is operated as follows.
First, when respiratory airflow with a certain flow rate is allowed to flow into a respiratory tube of a pneumotachometer by operating a vacuum pump, attractive force is generated by the vacuum pump within a glass tube which is arranged perpendicularly to the respiratory tube to communicate with the vacuum pump. The attractive force lifts a pendulum in the glass tube. At this time, airflow is generated between the pendulum and the wall of the glass tube, so the flow of the airflow moving in the tube is constant.
In this case, the pendulum ascends to a height at which the weight of the pendulum becomes equal to the attractive force, and the attractive force is proportional to the flow generated by the vacuum pump, so the height of the pendulum is proportional to the flow, that is, Hxe2x88x9dF. Further, the airflow with the same flow as that of the respiratory tube goes into the pneumotachometer to generate a differential pressure xcex94P.
Thereafter, if multivariate regression analysis adopting Equation [1] as a model equation is performed after several data pairs of xcex94P and F are measured by varying the attractive force of the vacuum pump to desired intensities, the measuring characteristic coefficients R0 and R1 which best satisfy the measured pairs of xcex94P and F can be obtained. Since the R0 and R1 are determined, the measuring characteristic coefficients R0 and R1 are applied to Equation [2], thus measuring new airflow, that is, respiratory flow. In this case, the determined measuring characteristics are characteristics under steady flow, that is, static characteristics.
However, since the measuring characteristics are static characteristics, they are obtained under the measuring environments differing greatly from dynamic airflow in which flow is instantaneously changed, like the respiratory airflow of the human body.
In order to compensate for the static characteristics, the American Thoracic Society (ATS) recommends that a 3 L syringe similar to the volume of the lungs of the human body is connected to a pneumotachometer, and the respiratory flow of the human body is simulated by manually reciprocating the handle of the syringe (refer to Am. J. Respir. Crit. Care Med. Vol. 152: 1107-1136, 1995).
All of airflows generated by the syringe flow into the pneumotachometer, and the volume of the syringe is uniformly 3 L. Therefore, if the flow signal measured by the pneumotachometer is integrated, the integrated result must be 3 L. However, since the measuring characteristic coefficients Ro and R1 which are previously calculated may not be ideal, the following Equation [3] is obtained if a proportional coefficient S is introduced and formulated.                     S        =                                            V              C                                      ∫                                                F                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                              =                      3                          ∫                                                F                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        [        3        ]            
In Equation [3], VC is the volume of the syringe, and the proportional coefficient S corrects the integrated value of the airflow to be VC=3 L by scaling the measured airflow at a predetermined rate regardless of the instantaneous value of the airflow.
The user of a respiratory airflow sensor measures respiratory airflow by using a mean value {overscore (s)} of the values of the proportional coefficient S obtained by repeatedly performing manual reciprocation of the syringe as an ultimate correction coefficient.
This method is widely used because of the convenience of the user""s ability to calibrate the pneumotachometer without an additional device except for the syringe. Generally, this method is carried out such that the user derives the following Equation [4], which is an ultimate characteristic equation, by further obtaining the mean value {overscore (s)} of the values of the proportional coefficient S, besides the measuring characteristic coefficients R0 and R1 determined by a manufacturing company, and then measures respiratory flow.                     F        =                              s            _                    ·                                                                                          R                    0                    2                                    +                                      4                    ⁢                                                                  R                        1                                            ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    P                                                              -                              R                0                                                    2              ⁢                              R                1                                                                        [        4        ]            
However, since Equation [1] is an experimentally-derived characteristic equation, Equation [4], which is derived from Equation [1], cannot provide the same accuracy for flow values of all airflows. Referring to fluid resistor-flow characteristics shown in FIG. 8, it can be seen that random errors are always present along linear flow values.
Therefore, errors incapable of being expressed by equations according to flow values are generated, and they overlap each other in the case of commercialized respiratory flow measuring devices, such that volume measurement error reaches 8% maximally (Journal of the Korean Society of Medical Informatics Vol. 6, pp 67-78, 2000).
Further, in a method of calibrating the respiratory flow measuring device by calculating the proportional coefficient S using the syringe after obtaining R0 and R1 through the static characteristic measuring experiment, R0 and R1 are not accurate with respect to flow values of all of airflows as described above, so the proportional coefficient S obtained by the Equation [3] is not constant. Further, since the same coefficients determined under the static environments are actually used under dynamic environments, the distortion of the values of the proportional coefficient S would be further increased.
In fact, as the result of a syringe experiment carried out by using the same coefficients obtained in static characteristic experiments of a pneumotachometer disclosed in U.S. Pat. No. 6,090,049, error which is not negligible is observed (relative error of maximum 7.5%), as shown in FIG. 9. Even though this error is corrected by Equations [3] and [4], the values of the proportional coefficient S are not constant and vary according to the characteristics of airflows.
Further, the conventional method of correcting measuring characteristics for respiratory flow is problematic in that, since measuring characteristics are inevitably uniformly corrected using a single proportional coefficient S without regard to the relative ratio of the characteristic coefficients R0 and R1, the user cannot correct measuring characteristics if the proportional relationship between R0 and R1 is varied due to the measuring environment, or if there is original error when R0 and R1 were determined.
Consequently, the measuring characteristic correction method and device is problematic in that, since it includes the cause of the inevitable measurement error, and requires a device for generating certain flow and a rotometer, measuring characteristic correction is excessively complicated and high cost is required, thus greatly decreasing accuracy and economic efficiency.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a system, which can conveniently and inexpensively obtain measuring characteristic correction factors, enabling a respiratory flow measuring device to more accurately measure respiratory flow compared with a conventional respiratory flow measuring device when respiratory flow is measured using the respiratory flow measuring device that utilizes static pressures, such as a pneumotachometer.
Another object of the present invention is to provide a method of obtaining measuring characteristic correction factors used in respiratory flow measurement while eliminating measurement error due to differences between characteristic equation calculating environments (static characteristics) and correcting environments (dynamic airflow), which is inevitably generated in a conventional respiratory flow measuring characteristic correction method.
In order to accomplish the above objects, the present invention provides a system for obtaining measuring characteristic correction factors for a respiratory flow measuring device using a static pressure difference, comprising a syringe connected to a respiratory tube of the respiratory flow measuring device through a duct line; a detecting unit for detecting volume variation within the syringe and outputting the detected result as a voltage signal; an analog/digital (A/D) converter for converting analog signals including a volume variation signal detected as the voltage signal by the detecting unit and a differential pressure variation signal detected as a voltage signal by a differential pressure sensor connected to the respiratory flow sensor of the respiratory flow measuring device into digital signals; and a computer for collecting the digital signals outputted from the A/D converter to calculate measuring characteristic correction factors.
Further, the present invention provides a method of obtaining measuring characteristic correction factors required for respiratory flow measurement using a measuring characteristic correction factor obtaining system, comprising the step of calculating characteristic coefficients R0 and R1 of the above Equation [1] by applying an instantaneous flow value F(t) obtained by differentiating the volume variation signal V(t), converted by the A/D converter, and the differential pressure variation signal xcex94P(t), converted by the A/D converter, to the following Equation [8].
Further, the measuring characteristic correction factor obtaining method of the present invention comprises the steps of drawing up a histogram according to strokes using differential pressure variation signal information accumulated in a manual reciprocation experiment of the syringe to calculate a volume measurement value after calculating the measuring characteristic coefficients R0 and R1, obtaining the ratio Sk of an actual volume value to the volume measurement value, and calculating a correction coefficient gi for correcting the volume measurement value according to respiratory flow values by applying the ratio Sk to the following Equation [23].